The dialkyl carbonates are precursors of bis-phenol-A-polycarbonate, a polymer known for its wide range of uses based upon its characteristics of transperancy, shock resistance and processability. They are important intermediates for the synthesis of fine chemicals, pharmaceuticals and plastics. Dialkyl carbonates also find applications as synthetic lubricants, solvents, plasticizes and monomers for organic glass. They are efficient alkylating agents and the processes using them are more eco-friendly compared to the conventional alkylation processes.
Conventionally, dialkyl carbonates and dimethyl carbonate in particular are synthesized using toxic chemicals like phosgene (COCl2). Alternatively, they can be synthesized by oxidative carbonylation (CO+O2) route using CuCl-base catalysts at high reaction temperatures. But this route is hazardous and produces corrosive hydrochloric acid. They are also produced from methanol by reacting with NO+CO+O2 in the presence of a Pd/C catalyst. This route also bears a potential explosion problem. Toxic NO is main problem in the carbonylation of intermediate methylnitrile in this route (Encyclopedia of Chemical Processing and Design, Vol 40, Ed. by J. J. McKetta and W. A. Cunningham, Marcel Dekker Inc., New York, 1992, and Ulmann's encyclopedia of Industrial Chemistry, Vol. A 21, Ed. by B. Elvers, S. Hawkins and G. Schulz, 5th ed. VCH Verlagsgesellschaft, mbH, Germany 1992).
Synthesis of dialkyl carbonates by transesterification of cyclic carbonates with corresponding alcohols is an eco-friendly, non-toxic route. In recent times dialkyl carbonates especially dimethyl carbonate and dipropyl carbonates are commercially synthesized by this technology (Filtration Industry Analyst 1999 (Issue No. 27, June 1999) 2 and S. Fukuoka, M. Kawamura, K. Komiya, M. Tojo, H. Hachiya, K. Hasegawa, M. Aminaka, H. Okamoto, I. Fukawa, S. Konno, Green Chem. 5 (2003) 497). Other than mineral acids and alkali bases, compounds like metal alkoxides (aluminum isopropoxide, tetraalkoxytitanium, (RO)Cu(PPh3)n, PdMe(OCHCF3Ph(dpe)), organotin alkoxides etc.), non-ionic bases (amines, dimethylaminopyridine, guanidines etc.) and lipase enzymes are known to catalyze the transformation of cyclic carbonates to dialkyl carbonates. But these catalysts possess the drawback of unstability or difficulty in separating and reuse (J. Otera, Chem. Rev. 93 (1993) 1449).
There were efforts toward developing solid catalysts for preparing dialkyl carbonates. Tatsumi et al. (Chem. Commun. Year 1996, page 2281) reported the synthesis of dimethyl carbonates from ethylene carbonate and methanol using K-TS-1 as a solid base catalyst. The transesterification of dimethyl oxalate with phenol was reported recently by Ma et al., (Fuel Proc. Tech. Vol. 83, Year 2003, page 275). Srivatsava et al reported the application of titanosilicate catalysts (Catal. Today Vol. 93, Year 2004, page 127). In all these applications the efficiency of the solid catalysts and yield of dialkyl carbonate is rather low. Wei et al (Green Chem. Vol. 5, Year 2003, page 343; Catalysis of Organic Reactions Ed. By D. G. Morrell, Marcel and Decker Inc., New York, Year 2003, Chapter 58, page 659), Chu et al (Inorg. Chim. Acta Vol. 307, Year 2000, page 131), Watanabe and Tatsumi (Microporous Mesoporous Mater. Vol. 22, Year 1998, page 399) and Fang and Xiao (Separation and Purification Tech. Vol. 34, Year 2004, page 255) report the use of solid catalysts but yield of dialkyl carbonate is low (50 mol %). There were reports to synthesize dialkyl carbonates in a single-step process by reacting epoxides, CO2 and alcohol over solid catalysts (Bhanage et al., Green Chem. Vol. 5, Year 2003, page 71; Appl. Catal. A: Gen Vol. 219, Year 2001, page 259; Chang et al., Appl. Catal. A: Gen Vol. 263, Year 2004, page 179; Jiang and Yang Catal. Lett. Vol. 95, Year 2004, page 127) but the yields of dialkyl carbonates are very poor. U.S. Pat. No. 6,835,858 describes the preparation of organic carbonates over Zn supported catalysts at 10° C., 25 bar pressure and space velocity WHSV=5 g/g/h. But the yield of dialkyl carbonate is low and leaching of Zn from the solid was detected. U.S. Pat. No. 6,479,689 tells a method for continuously producing of dialkyl carbonate and diol wherein dialkyl carbonate yields of 99% could be obtained but then catalyst separation was an issue. U.S. Pat. No. 6,392,078 details the synthesis of dimethyl carbonate from urea and methanol using homogenous tin alkoxide catalyst. U.S. Pat. No. 6,407,279 describes the synthesis from epoxide, monohydric alcohol and CO2 using a homogeneous and heterogeneous catalyst. In other words, the prior art processes have one or more of the following disadvantages: (1) difficulties in catalyst separation, lower efficiency of the catalyst, catalyst stability during reaction, lower yields of dialkyl carbonates etc.
The present invention deals with a process, which eliminates all the above said drawbacks of the prior-art processes. It deals with production of dialkyl carbonates which comprises reacting of a cyclic carbonate with an alcohol over a solid, reusable double metal cyanide catalyst. One of the metals of the double metal cyanide catalyst is a Lewis acid metal ion such a Zn2+ while the other is a basic metal ion such as Fe. Co-existence of Zn and Fe in the active site linking through cyano-bridges makes catalyst efficient to transform cyclic carbonates into dialkyl carbonates. The catalyst could be separated easily by centrifugation or simple filtration and reused in several recycling experiments with little loss in activity/selectivity. Most importantly, the catalyst is highly efficient and only a small amount (˜5 wt % of cyclic carbonate) is needed to carryout the reaction. The process is atom-efficient and the reaction conditions like temperature and pressure are moderate.